I posted a few times already about Allen et al’s paper on the Trillionth Tonne, ever since I saw Chris Jones present it at the EGU meeting in April. Basically, the work gets to the heart of the global challenge. If we want to hold temperatures below a 2°C rise, the key factor is not how much we burn in fossil fuels each year, but the cumulative emissions over centuries (because once we release carbon molecules from being buried under the ground, they tend to stay in the carbon cycle for centuries).

Allen et. al. did a probablistic analysis, and found that cumulative emissions of about 1 trillion tonnes of carbon give us a most likely peak temperature rise of 2ºC (with a 90% confidence interval of 1.3 – 3.9°C). We’ve burnt about half of this total since the beginning of the industrial revolution, so basically, we mustn’t burn more than another 1/2 trillion tonnes. We’ll burn through that in less than 30 years at current emissions growth rates. Clearly, we can’t keep burning fossil fuels at the current rate and then just stop on a dime when we get to a trillion tonnes. We have to follow a reduction curve that gets us reducing emissions steadily over the next 50-60 years, until we get to zero net emissions. (One implication of this analysis is that a large amount of existing oil and coal reserves have to stay buried in the ground, which will be hard to ensure given how much money there is to be made in digging it up and selling it).

Anyway, there’s now a website with a set of counters to show how well we’re doing: Trillionthtonne.org. Er, not so well right now, actually.

Howard – many thanks! Am enjoying reading through the People and Place articles. I did try a few months back to see if I could figure out what a “fair share” of the remaining 1/2 trillion tonnes would be for each person on the planet, but I got bogged down in the mathematics of how long each person is alive for, and where their lifespan falls within the target emissions curve. There’s something to be said for the argument that previous generations didn’t know the consequences of their emissions (and the rapid development that it enabled has been very good for a large number of people in the developed world). But the equity issue going forward is of paramount importance, and I do think the idea of a total carbon budget helps to crystalize the issue. Have you written anything about the “contraction and convergence” idea?

Hi Steve – not about C+C per se. I did invite an article by Paul Baer on the Greenhouse Development Rights framework. GDRs would also force a convergence of emissions. In the article, Paul compares the rights-based GDR approach with the equity-based, “one person, one share” approach: http://bit.ly/1a3Nj7 .